[HtmlTargetElement(Attributes = AttributeName)]
public class InDomIfTagHelper : TagHelper
{
    private const string AttributeName = "idi,in-dom-if";

    [HtmlAttributeName(AttributeName)]
    public bool InDomIf { get; set; }

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        base.Process(context, output);
        if (!InDomIf)
        {
            output.SuppressOutput();
        }
    }
}

<li idi='false'>content</li>
<li in-dom-if="true">content2</li>